1. Field of the Invention
This invention relates to a process for preparing a selective permeable membrane. More particularly it relates to a process for preparing a selective permeable polyimide membrane which is suitable for use in separation of organic liquid mixtures as well as for aqueous liquid mixtures.
2. Description of the Prior Art
Membranes which can selectively permeate specific components in a solution or a liquid mixture such as an emulsion and a suspension are generally called "selective permeable membranes". The membrane-separation treatment utilizing such characteristics has hitherto been employed only for use with aqueous liquid mixtures. The reason for this is that permeable membranes made from celluloses such as cellophane, cellulose acetate, or cellulose nitrate generally have no resistance to organic solvents, and permeable membranes prepared from synthetic resins such as polyvinyl chloride, polypropylene and polystyrene are soluble in organic solvents, or at least swell therein if not soluble, and thus those membranes cannot be used for membrane-separation treatments.
However, membrane-separation treatments have been highly desired not only for aqueous liquid mixtures, but also for organic liquid mixtures, typically organic solutions, and development of such selective permeable membranes has been sought.
Fluoro resins, exemplified by polytetrafluoroethylene, are not always suitable for preparing such selective permeable membranes although they generally have excellent organic solvent resistance because the formation of such selective permeable membranes having fine pores is difficult to achieve, and the resins are very expensive.
Therefore, it has recently been proposed to produce selective permeable membranes using aromatic polyimides having organic solvent resistance. These aromatic polyimides have good organic solvent resistance as they are almost insoluble in organic solvents, but this causes problems in membrane formation, which is complicated and requires special techniques and many steps.
For example, Strathmann, Desalination, Vol. 26, p. 85 (1978) discloses a process for the production of aromatic polyimides which comprises reacting aromatic tetracarboxylic acid anhydride and aromatic diamine to obtain a polyamide acid soluble in organic solvents, flowcasting a solution of the polyamide acid on to a suitable support to provide a membrane structure, and then dehydrocyclizing the polyamide acid by chemical reaction or heat treatment. This process requires the additional procedure of converting the amide acid structure into the imide structure, and this procedure does not always proceed smoothly.